Embodiments of the invention relate generally to an electric power grid and more specifically to distribution protection system in the power grid.
Distributed generation generates electricity from many small energy sources such as photovoltaic cells and fuel cells. Instead of producing power using remote and large generator units, power is generated using a large number of small distributed generators to meet the local load demand. These small generators are interconnected to the grid at medium or low voltage levels. Solar PV as an example is increasingly being connected at low voltage levels as roof-top installations.
Generally, the distribution network topology, control and protection are designed assuming that power is flowing in one direction; from substation to loads. However, the presence of distributed generation may change both the magnitude and direction of power flow in the distribution network or the distribution system. The variability in distributed generation such as the intermittency in renewable generation causes the system operating conditions to vary frequently. For example, a loss or gain of one or more distributed generators may cause the feeder voltage to fluctuate or even violate the desired range. Without coordination, these changes may trigger the false tripping of protective relays including over-current, over-voltage or under-voltage relays.
In addition, disturbances in the distribution system may affect the operation of distributed generators. For example, IEEE Standard 1547 stipulates that when any voltage of a distributed generator bus is outside a given range, the distributed generator shall cease to energize the feeder (i.e. shut down by tripping offline) within a specified clearing time. The clearing time is the time between the start of a disturbance condition and the ceasing of the distributed generator to energize the feeder. The tripping of one distributed generator may deteriorate the voltage profile further and potentially result in cascading tripping of other distributed generators.
Another issue with connection of distributed generators is that it changes the fault current of the distribution system. In other words, when you connect a distributed generator to the distribution system it will contribute to the fault current based on the power it is generating. This can lead to a failure of protection systems to detect faults when there are high levels of distributed generation. One of the approaches to solve this problem is to adaptively change relay settings in coordination with changes in output power of the distributed generation. However, with this approach, the relay settings may not get updated as fast as the output of the distributed generation changes. A potential problem is that a sudden loss of distributed generator under full load may result in the tripping of the over-current relay when the relay set point is reduced to a very low level. Thus, large scale penetration of distributed generation will reduce the effectiveness of protection schemes either through reducing the detection of faults, of creating false trips in response to the loss of distributed generators.
Therefore, there is a need for an improved protection system and method to address one or more aforementioned issues.